Environmental Engineering Reference
In-Depth Information
Figure 4: Savonius turbines used to assist ventilation and generate electricity on
the Council House 2 (CH2) landmark building in Melbourne, Australia
(photographs
copyright Pauline Anastasiou).
There have been many studies of the performance of the Savonius rotor, how-
ever, it would appear that the coeffi cient of performance, C p , is modest. Modi and
Fernando [7], for example, tested a wide range of Savonius rotor geometries in a
wind tunnel with variations in the degree of overlap and separation of the blades
and rotor aspect ratio. Modi and Fernando also carried out important tests to
determine the blockage effect of the turbine in the wind tunnel. Thus, they were
able to extrapolate their results to estimate the performance of Savonius rotors
under unconfi ned conditions. Their conclusion was that the best coeffi cient of
performance of a geometrically optimised Savonius rotor was likely to be C p,max
0.7. Ushiyama and Nagai [8] carried out uncon-
fi ned tests with various Savonius rotors located downstream of the exit of a wind
tunnel. The maximum rotor coeffi cient of performance of C p,max
0.3 at a tip speed ratio of l
0.23 at a tip
speed ratio of l
1.0 was found to be less than that reported by Modi and Fer-
nando although it was not made clear whether the effects of bearing frictional
losses were accounted for in the experiments. More recent studies have also been
conducted on a number of geometric variations including stacking rotors one
above the other and Rahai has reported on optimisation of the Savonius design
using CFD analysis [9].
2.2.2 Darrieus turbines
In 1931 the invention by Darrieus [10] of his rotor with a high tip speed ratio
opened up new opportunities for VAWTs in regards to electricity generation. The
fundamental step forward made by Darrieus was to provide a means of raising the
velocity of the VAWT blades signifi cantly above the freestream wind velocity so
that lift forces could be used to signifi cantly improve the coeffi cient of performance
of VAWTs over previous designs based primarily on drag. Darrieus also foresaw a
number of embodiments of his fundamental idea that would be trialled at large scale
many decades later. These included use of both curved-blade (Fig. 5a ) and straight-
blade versions of his rotor. He also proposed options for active control of the pitch
of the blades relative to the rotor as a whole, so as to optimise the angle of attack
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